Effect of oxidation with HNO₃ on the heterogeneous Fenton-like catalytic properties of g-C₃N₄

Full text: Metal-free catalysts have been considered an alternative to traditional metal-based compounds. Due to its advantages, such as the multiple superficial functionalities, the g-C₃N₄ is a great candidate for the development of new catalysts that are more efficient and aligned with the concept of sustainability. We recently showed that g-C₃N₄ , prepared in an open atmosphere, can promote the formation of •OH in the presence of H₂O₂ (heterogeneous Fenton). In this work we investigate the effect of oxidation of g-C₃N₄ with HNO₃. The g-C₃N₄ was synthesized by polycondensation of melamine and the material obtained was subjected to acid treatment with HNO₃ in concentrations of 1, 3 and 5M for 2h at 80 °C. All samples were characterized by different techniques, including DRX, FTIR and SEM. The catalytic activity was evaluated through the discoloration of the indigo carmine dye (10 mg/L) in the presence of 15 mg of g-C₃N₄ and 25 μL of H₂O₂. The investigation of the catalytic activity showed that the sample g-C₃N₄-3M showed the best rate of discoloration, degrading 34% of the dye after 90 minutes of reaction. While the sample without acid treatment showed an efficiency of 20%. These results suggest that functional groups oxygenated with intermediate NO_x, C-OH and/or N-OH, may be responsible for the H₂O₂ reduction reaction forming •OH. The results presented showed that the heterogeneous Fenton-like catalytic properties of g-C₃N₄ were enhanced. However, precise control of reaction conditions is necessary to obtain a material with a high concentration of these functional groups with intermediate NO_x. (author)